Article
pubs.acs.org/JPCB
Heterogeneous Nucleation of Giant Bubbles from a Langmuir
Monolayer in a Laser Focus
Jürgen Gewinner and Thomas M. Fischer*
Institut für Experimentalphysik, Universität Bayreuth, 95440 Bayreuth, Germany
S Supporting Information
*
ABSTRACT: Evidence is shown that spherical structures of methyloctadecaoate nucleated
from a Langmuir monolayer in a laser focus are giant multilamellar bubbles. The bubbles
remain stable in the laser focus but respread to the monolayer outside of the focus. Bubbles
coalesce when brought into contact. The coalescence is accompanied by a volume increase and
area decrease of the fused bubble as compared to the original pair of bubbles. Bubbles deviate
from spherical and are compressed along the flow direction of the surrounding monolayer
when they are advected versus and trapped at phase boundaries of the monolayer.
■
INTRODUCTION
Langmuir monolayers at the air−water interface exhibit a rich
variety of different phases as a function of their surface density
and as a function of temperature.1 Their insolubility in both
polar and apolar liquids usually forces them to remain at the
air−water interface and avoid the bulk of the water. However,
compression beyond the equilibrium spreading pressure
renders the interfacial arrangement of the molecules metastable.
As a result the rate of formation of three-dimensional structures
on top or below the monolayer increases with the amount of
supercompression of the film. The resulting collapse structures
seem to depend on the monolayer equilibrium phases prior to
collapse, and they vary with the compression rate. They
therefore also depend on transport properties such as the
viscoelastic properties of the film. The collapse structures might
consist of bilayers formed on top of the monolayer2 or by
structures that form in the water below the monolayer. Folds,3
tubes,4 buds,5 or small vesicles,6,7 but also irregular structures8
are examples of structures forming at the aqueous side of the
collapsing monolayer. These structures can be reversibly or
irreversibly nucleated upon compression. Reversible collapse is
a very important mechanism occurring at the gas-exchange
interface in the lung−alveoli during the breathing cycle. This
has not only triggered the experimental research on monolayer
collapse but has also led to the theoretical work of Lu et al.9 and
to simulations by Baoukina et al.10 of monolayer collapse. Lu et
al.9 could show that the formation of elliptical folds occurs at
surface pressures far below the surface tension of water because
of the binding energy between the leaflets of the bilayers within
the fold. Baoukina et al.10 used simulations to confirm these
findings but also to predict the formation of semivesicles
remaining bound to the monolayer or to detached full vesicles.
Recent theoretical work has suggested folds of shapes
modulated along the fold by a soft Goldstone mode.11
On a macroscopic scale, nucleation of collapse structures is
homogeneous because the same thermodynamic conditions
© 2013 American Chemical Society
hold throughout the entire monolayer. Microscopic techniques
reveal only a small region of the entire monolayer. Collapse
generically happens outside of the field of view and can be
visualized only indirectly from intermittent jerks observed in
the motion of the monolayer in the field of view.12,13 However,
the pioneering work of Angelova, Vollhard, and Ionov8 showed
that collapse structures preferentially nucleate at the border of
condensed monolayer domains. Hence, on a mesoscopic scale,
nucleation is heterogeneous. This brings about the idea to
heterogeneously induce the nucleation of monolayer collapse
by locally perturbing the monolayer at a single well-defined
location.
Vesicles are the most symmetric among all collapse
structures. They can be formed by more efficient mechanisms
than monolayer collapse and have been investigated intensely.
When put under stress, vesicles may connect to other structures
via tubes14−16 that coexist with the vesicles under equilibrium
conditions. Bubbles are the analogue of vesicles on the gaseous
side of the air−water interface. Soap bubbles are common
structures forming from the aqueous phase when soluble
surfactants are present. Circular bubbles of thermotropic
smectic liquid crystals are bubbles formed in the absence of
water from water-insoluble molecules.17−20 The formation of
bubbles from water-insoluble surfactants on the other hand is
less common.
The aim of this work is to present a new technique that
allows nucleating collapse structures in the focus of a laser
positioned in the field of view of the microscope. An analysis of
the intensity profile, coalescence behavior, and hydrodynamic
flow of the spherical collapse structures lets us conclude that
the collapse structure is an insoluble surfactant bubble that
remains connected to the monolayer rather than a vesicle.
Received: July 23, 2013
Revised: October 25, 2013
Published: November 7, 2013
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The Journal of Physical Chemistry B
Article
Figure 1. (left) Fluoresence microscopy image of a circular collapse structure observed near the laser focus. (right) Intensity profile of the structure
to the left. The solid line is a fit with the intensity profile of a spherical shell plus a background noise intensity. A video (jp407291a_si_001.mpg) of
the nucleation process of the structure is provided in the Supporting Information.
■
MATERIALS AND METHODS
Methyloctadecanoate was purchased from Sigma Aldrich. The
surfactant was dissolved in chloroform (about 1 mM) and
fluorescently labeled with 1 mol of 1,2-dihexadecanoyl-snglycero-3-phosphoethanolamine and triethylammonium salt
(Texas Red DHPE) purchased from Molecular Probes
(Eugene, Oregon, U.S.A.). The surfactant was spread at the
air−water interface. The subphase was pure water (Millipore
Milli-Q 18 MΩ cm). The setup for studying collapse
phenomena has been described in detail elsewhere.21 Briefly,
it consists of a home-built Langmuir trough placed on the stage
of an inverted fluorescence microscope (Zeiss-Axiovert 135)
with a 100× water immersive objective. The temperature of the
trough can be controlled precisely. An IR laser beam (λ = 1064
nm, P = 2 mW−10 W) was used to locally heat the monolayer
in the focus of the objective. The light is partially absorbed by
the subphase and heats the monolayer locally around the focus.
■
lateral distance from the center of the circular structure on the
right-hand side of Figure 2. For a spherical shell of radius R of a
Figure 2. Superposition of two fluorescence microscope images of two
spherical collapse structures (red) just prior to coalescence and the
resulting structure (blue) after coalescence. The scale bar is 50 μm. A
video (jp407291a_si_002.mpg) of the coalescence is provided in the
Supporting Information.
RESULTS
In previous work,22,23 it was observed that upon focusing an IR
laser on a methyloctadecanoate monolayer in the liquid
condensed phase, at a laser power higher than a critical value
of about 2 W, a radial thermocapillary flow of the surface
toward the center sets in, resulting in local collapse of the
monolayer. The collapse structure in methyloctadecanoate
depends on the surface temperature, surface pressure, and laser
power of the IR laser. Here, we focus on spherical structures
forming upon collapse and show that they are bubbles tethered
to the monolayer.
Our experiments are performed at an area per molecule
below 19 Å2/molecule and at a temperature of T = 28 °C. At
these conditions, the monolayer has already globally collapsed
via cracks; however, no homogeneous nucleation of 3d
structures is observed in the field of view with the fluorescence
microscope. Upon increasing the IR laser power beyond the
threshold power of Pc ≈ 2 W, we observe a radial inward flow
of the monolayer toward the laser focus. When we decrease the
area per molecule, the threshold power for collapse usually
decreases. Collapse structures occur at high laser powers and
low area per molecule. In the center, three-dimensional
aggregates form, which coexist with highly symmetric circular
structures. In the left part of Figure 1, we show a fluorescence
microscope image of such a circular structure. The measured
intensity profile of the circular structure is plotted versus the
randomly oriented fluorescence dye, we would expect an
intensity profile I(x,y) ∝ (R2 − x2 − y2)−1/2 because the
fluorescence light eminates only from the shell of the sphere
and not from the interior of the sphere. The solid line in the
figure corresponds to such an intensity profile. We observe
circular collapse structures with radii ranging from 1 to 40 μm.
It is possible to nucleate two or more spherical collapse
structures in the laser focus. A collection of two spheres
(colored red) is shown in Figure 2. Eventually, two spheres of
radii R1i and R2i coalesce to form a third single spherical
structure of radius Rf (colored blue). Assuming a spherical
shape of the particle, we can compute the area A = 4πR2 and
volume V = 4π/3R3. We observe an increase of volume ΔV/V =
(Vf − V1i − V2i)/Vf ≈ 10% ± 7% upon fusion, while the
absolute area shrinks ΔA/A = (Af − A1i − A2i)/Af ≈ −10% ±
7%. Figure 2 shows two spheres prior to coalescence (red) and
the resulting final single sphere (blue).
When we decrease the laser power, surfactant molecules
leave the surface of the sphere and are respread to the
monolayer. In Figure 3, we show fluorescence images of the
respreading when the laser power is reduced. A corona of
respread monolayer surrounds the spherical object.
Tethering and Resistance to Flow. The monolayer in the
focus of the IR laser is often subject to characteristic flow. The
flow profile of the monolayer can be tracked via the texture of
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bubble, the interior monolayer of the bubble would be oriented
with the alkyl chains toward the air. We find it difficult to
explain the volume increase if the spherical objects are vesicles.
If the spherical objects are bubbles, our reasoning is as follows.
Bubbles cannot be created without the IR laser because without
the laser, it is not possible to create sufficient internal pressure
in the bubble interior to stabilize the bubble. We hence assume
that the internal pressure in a bubble is the air pressure inside of
the gas bubble plus an additional pressure created by the laser
pi = piair + p laser
(1)
In mechanical equilibrium, the internal pressure is balanced by
the outside air pressure pair
o plus the Laplace pressure produced
by the lipid film of the bubble
4σ
pi = poair +
(2)
R
Figure 3. Superposition of fluorescence images of a spherical collapse
structure for different lag times after reducing the laser power
(different times are colored differently). No monolayer has respreads
right after the reduction (brown), a growing corona of the respreading
monolayer surrounds the spherical structure as time progresses, while
the size of the spherical structure shrinks a little (pink). A video
(jp407291a_si_003.mpg) of the monolayer expulsion is provided in
the Supporting Information.
Here, σ denotes the interfacial tension of one side of the bubble
film, and R is the radius of the bubble. When the bubble is
nucleated by the laser, the inside air pressure initially cannot
exceed the outside air pressure, and we can estimate the laser
pressure necessary for nucleation of the bubbles by the Laplace
pressure in the smallest bubbles observed.
4σ
p laser ≈
R min
(3)
the monolayer. In Figure 4, we depict the monolayer and
several nucleated three-dimensional spherical structures resid-
Here, Rmin ≈ 1 μm is the smallest radius of bubbles observed in
the experiment. We assume that the inside air is at a constant
temperature, and an ideal gas behavior
piair Vi = NkT
(4)
holds for the air inside of the bubble.
Upon coalescence, we assume both bubbles to be
impermeable for the air of the outside, such that the number
of air molecules in the final bubble equals the number of air
molecules inside of the initial pair of bubbles, and
piair
V + piair
V = piair
V
,1i 1i
,2i 2i
,f f
(5)
where we have indices i1,i2 for the initial bubbles and f for the
final bubble. Because the radius of the final bubble is larger than
that of the initial bubble, the inside pressure will be reduced by
the decrease of the Laplace pressure such that the inside air can
expand.
Inserting eqs 1−4 into eq 5, we arrive at
Figure 4. Spherical collapse structures stationary positioned inside of
the flow of the surrounding monolayer. The collapse structure are
darker at the front facing the flow than that at the rear. The scale bar is
50 μm. A video (jp407291a_si_004.mpg) of the collapse structures in
the flow is provided in the Supporting Information.
⎛
⎛ air
4σ
4σ ⎞
4σ
4σ ⎞
−
−
⎟V2i
⎟V1i + ⎜p0air +
⎜p0 +
R1i
R min ⎠
R 2i
R min ⎠
⎝
⎝
ing in the flow. The two-dimensional flow profile is shown as
yellow arrows and resembles that of capillary flow around a
two-dimensional melted gaseous domain recently observed by
Muruganathan et al.24 The spherical structures are stationary
and located at the rear of the melted moving phase close to the
boundary to the quiescent monolayer phase. It is evident that
the spherical structures are brighter at their rear than at the
front that faces the surrounding monolayer flow.
Resolving eq 6 for the interfacial tension, we find
DISCUSSION
From Figure 1, we conclude that the circular object is a
spherical shell of insoluble surfactants. In principle, it could be
either a vesicle or a bubble. In one case, the interior lipid
monolayer would be oriented with the head groups toward the
internal aqueous phase. If the spherical shell is an air-filled
The quantities on the right-hand side can be extracted from the
experiments, and we find an interfacial tension of σ ≈ 25 mN/
m of the surfactant−air interface corresponding to a surface
pressure of roughly π ≈ 50 mN/m that corresponds reasonably
well with the collapse pressure πc ≈ 40 mN/m of the
methyloctadecanoate monolayer at the air−water interface.25
⎛
4σ
4σ ⎞
= ⎜p0air +
−
⎟Vf
Rf
R min ⎠
⎝
σ=
■
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p0air R min
4
(6)
1
(1 −
ΔAR min
3ΔV
)
(7)
dx.doi.org/10.1021/jp407291a | J. Phys. Chem. B 2013, 117, 14749−14753
The Journal of Physical Chemistry B
Article
material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 5 shows the surface tension extracted from the
experiments as a function of the coalesced bubble.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: thomas.fi[email protected].
Notes
The authors declare no competing financial interest.
■
■
ACKNOWLEDGMENTS
This work has been supported by the German Science
Foundation via Grant Fi 548/11-1
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Figure 5. Surface tension of the bubbles versus the radius of the final
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When the laser is shut off, the bubble acts a reservoir for the
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air−water interface, and we observe a respread surface (Figure
3) that exceeds the area Af = πR2f of the bubble without
significant reduction in size of the bubble. We conclude that the
bubble consists of a multilamellar film of insoluble surfactants.
The fact that the bubble can be deformed by flow shows that
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water−air surface. It also shows that there is a monolayer phase
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across the bubble.
■
CONCLUSIONS
In conclusion, we have proven that the circular collapse
structure is a multilamellar bubble filled with air by the IR laser
that is connected to the air−water interface. The bubble is
stable only in the presence of the laser and is successively
stripped of bilayers when the laser power is reduced. The
bubble consists of a multilamellar bilayer of surfactants, and
defects in the bilayers can be produced by viscous forces that
lead to bubbles with different numbers of bilayers in the front
as compared to those in the rear of the bubble.
■
REFERENCES
ASSOCIATED CONTENT
S Supporting Information
*
Videos of the nucleation process of the structure (jp407291a_si_001.mpg), the coalescence (jp407291a_si_002.mpg), the
monolayer expulsion (jp407291a_si_003.mpg), and the
collapse structures in the flow (jp407291a_si_004.mpg). This
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The Journal of Physical Chemistry B
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